On Tue, Mar 12, 2013 at 12:14 AM, Boris Zbarsky <bzbar...@mit.edu> wrote:
> CSE can get rid of the redundant .data gets. Similarly, .data gets can be > loop-hoisted in many cases. > Doing COW based on page-faults is nicer anyway, but I don't know about the data structures of JS engines to know whether this is feasible. (For example, if an object in JS is preceded by a header that gets written by the engine now and then, it'll probably lie on the same page as the data, which would trigger an expensive fault each time.) I suppose padding the backing store so it doesn't share pages with anything else might be reasonable here: up to about 8k of waste on a system with 4kb pages. The cost of marking the pages read-only would only have to be paid when the copy-on-write action (eg. a call to putImageData) is actually made. Very small buffers could simply disable copy-on-write and always perform a copy, where the waste for padding is more significant and the benefits of avoiding a copy are smaller. (For what it's worth, marking a 128 MB buffer read-only in Linux with mprotect takes on the order of 3 microseconds on my typical desktop-class system. I don't know if Windows's VirtualProtect is slower.) On Tue, Mar 12, 2013 at 4:04 PM, Rik Cabanier <caban...@gmail.com> wrote: > > I don't think it is necessary to provide a createImageDataHD in this > > interface. The caller will know the devicePixelRatio and determine > > whether to generate high-DPI data. > > That probably won't work since it results in code that executes differently > on devices that are HD. > The difference between getImageData(HD) and putImageData(HD) is in the canvas operation, not the ImageData: it determines how pixels are scaled when being read out of and written into the canvas backing store. It doesn't apply to this API; ImageData objects don't know anything beyond their pixel data and dimensions. (Code executing differently on high-DPI devices is a bridge we've already crossed. getImageData scales pixels down from the device's pixel ratio; getImageDataHD doesn't, copying backing store pixels one to one.) There is no real reason why you could use it in both HD and non-HD APIs. > Rather, there's no reason you couldn't. You can definitely create an ImageData with getImageData and then pass it to putImageDataHD (which would cause the image to be scaled on devices with a pixel ratio other than 1, of course). -- Glenn Maynard
